Organic electroluminescent (EL) elements include a thin film containing a luminescent organic compound interposed between a cathode and an anode. Electrons and holes are injected into the thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons. The organic EL elements are characterized by plane light emission at a high luminance of about 100 to 10,000 cd/m.sup.2 with a low voltage of about 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
The organic EL elements, however, are undesirably short in effective life, less durable and less reliable because of the following factors.
(1) Physical changes of organic compounds: Growth of crystal domains renders the interface non-uniform, which causes deterioration of electric charge injection ability, short-circuiting and dielectric breakdown of the element. Particularly when a low molecular weight compound having a molecular weight of less than 500 is used, crystal grains develop and grow, substantially detracting from film quality. Even when the interface with ITO is rough, significant development and growth of crystal grains occur to lower luminous efficiency and allow current leakage, ceasing to emit light. Local dark spots are also formed. PA1 (2) Oxidation and stripping of the cathode: Although metals having a low work function such as Na, Mg and Al are used as the cathode in order to facilitate electron injection, these metals are reactive with oxygen and moisture in air. As a result, the cathode can be stripped from the organic compound layer, prohibiting electric charge injection. Particularly when a polymeric compound is applied by spin coating, the residual solvent and decomposed products resulting from film formation promote oxidative reaction of the electrodes which can be stripped to create local dark spots. PA1 (3) Low luminous efficiency and increased heat build-up: Since electric current is conducted across an organic compound, the organic compound must be placed under an electric field of high strength and cannot help heating. The heat causes melting, crystallization or decomposition of the organic compound, leading to deterioration or failure of the element. PA1 (4) Photochemical and electro-chemical changes of organic compound layers.
To solve problem (1), low molecular weight amorphous compounds and high molecular weight compounds have been studied. The low molecular weight compounds can be deposited by evaporation, but the resulting thin films are unstable. The high molecular weight compounds can form stable thin films, but suffer from a serious problem of application because they cannot be deposited by evaporation in forming a layer structure. Since they are coated as by spin coating, the residual solvent and impurities can be incorporated into the resulting film to invite substantial deterioration of the electrodes and high molecular weight compounds. High molecular weight compounds which can be deposited by evaporation were recently reported (see Fukuda et al., Preprint of the Japanese Polymer Society's 41st Annual Meeting in 1992, IL-29), but are still unsuccessful in providing emission at a practically acceptable luminance.
The transparent electrodes used heretofore are of ITO glass or the like because they must have a low surface resistivity of less than 10 to 30 .OMEGA./. However, observation under a scanning tunnel microscope (STM) or atomic force microscope (AFM) indicates irregularities of the order of 200 .ANG. on sputtered substrates and of the order of 400 .ANG. on EB evaporated substrates. There is also surface roughening due to damage during ITO patterning. Therefore, the prevailing situation is likely to promote crystallization of an organic thin film.
Solutions in this respect include provision of a phthalocyanine film on the ITO surface (see JP-A 295695/1988) and spin coating of polyarylene vinylene. However, the phthalocyanine is microcrystalline and is not always effective. The polyarylene vinylene can damage ITO with an acid generated upon conversion and promote oxidation of the electrodes by residual solvent or the like. A fill of polyarylene vinylene is non-uniform because of spin coating. All these fail to improve element reliability.
Therefore, there is a need for an organic compound which can solve the above-mentioned problems.
For the purpose of improving element performance, EL elements having a mix layer consisting of a mixture of two or more compounds having distinct functions were also proposed. For example, JP-A 250292/1990 discloses that a thin film of layered structure or a mix thin film prepared from an organic compound having hole transport and light emitting functions and another organic compound having an electron transport function is used as a light emitting layer for the purpose of improving luminance and durability. JP-A 291696/1990 discloses to use a thin film of a mixture of an organic compound having a hole transport function and a fluorescent organic compound having an electron transport function as a light emitting layer. JP-A 114197/1991 discloses to interpose a mix layer of a mixture of an electric charge injecting material and an organic fluorescent material between an electric charge injecting layer and a light emitting layer for the purpose of improving luminous efficiency and luminance. JP-A 190088/1991 discloses to interpose between a hole transport layer and/or an electron transport layer and an organic light emitting layer a mix layer containing the components of the opposed layers for the purpose of facilitating injection of holes and electrons into the light emitting layer. JP-A 334894/1992 discloses that when a plurality of organic compound layers are formed, a layer in which compounds of distinct functions are co-present is formed, for example, a layer containing a hole transporting luminescent material and a layer in which a hole transporting luminescent material and an electron transporting material are co-present are formed, thereby increasing luminance, providing a variety of emission color hues and improving durability. JP-A 182762/1993 discloses to form a mix layer of a mixture of a luminescent material and an electric charge injecting material between a light emitting layer and an electric charge injecting layer, thereby lowering the drive voltage. JP-A 289090/1991 discloses to form a thin film of a mixture of a hole conducting organic compound and an organic complex of a rare earth metal as a light emitting layer, achieving a narrow luminous spectrum, monochromaticity, and high conversion efficiency. JP-A 178487/1992 and 78655/1993 discloses high luminance full-color elements which are obtained by forming a thin film layer of a mixture of an organic charge material and an organic luminescent material as an organic luminescent thin film layer, thereby preventing concentration extinction and increasing the available range of luminescent material. Moreover, JP-A 357694/1992 discloses to form layers of graded structure in which a concentration gradient is provided between adjacent layers by components of respective layers, thereby lowering the drive voltage and improving durability.
Also organic compound layers doped with rubrene were proposed. Typical examples of known organic compound layers doped with rubrene are found in organic EL elements comprising a hole transport layer in the form of a film of a mixture of hydrazine derivatives and a light emitting layer of tris(8-quinolinolato)aluminum as organic compound layers wherein the hole transport layer is doped with rubrene or a half portion of the hole transport layer disposed on the organic interface and the entire light emitting layer are doped with rubrene. It was reported that in the element having the hole transport layer doped with rubrene, light emission takes place from both tris(8-quinolinolato)aluminum and rubrene and that in the element having a half portion of the hole transport layer and the light emitting layer doped with rubrene, luminous efficiency is improved and the increase of dark spots during shelf storage is suppressed. See Kanai, Yajima & Sato, Extended Abstracts of the 39th Spring Meeting, 1992 of The Japan Society of Applied Physics and Related Societies, 28p-Q-8 (1992) and Sato & Kanai, Preprint of Workshop 92 of the Japanese Research Association for Organic Electronics Materials (JOEM), 31 (1992). A hole transport layer of triphenyldiamine derivative (TPD) doped with rubrene was also proposed as having an improved luminance half-life. See Fujii, Sano, Fujita, Hamada & Shibata, Extended Abstracts of the 54th Autumn Meeting, 1993 of The Japan Society of Applied Physics, 29p-ZC-7 (1993).
Moreover, JP-A 207488/1990 discloses an element comprising a p-type inorganic semiconductor thin film layer and an organic compound thin film layer consisting essentially of rubrene, the element providing satisfactory luminance and stability thereof.
None of the foregoing EL elements are satisfactory in luminous life.